Methods: Electron treatments were planned for 58 patients on Eclipse (Version 10.0) using the eMC algorithm for two Varian TrueBeam linear accelerators (Linac-A and Linac-B). The dose calculations were based on patient anatomy (patient plan), and a solid-water phantom (QA plan). The calculation results were compared with physics measurement using a solid-water phantom and a pinpoint ion chamber.

Results: The monitor units as calculated on the QA plans were (2.1±2.0)% higher than the measurement on both accelerators, excluding outliers with small fields (3 cm or less) and extended SSD (above 105 cm). The difference of QA plan and measurement was consistent with the calculation uncertainty of the eMC algorithm (2%). The monitor units as calculated on the patient plans were (8.0±5.0)% (Linac-A) and (5.5±5.0)% (Linac-B) higher than the corresponding measurement. However, we observed that the eMC algorithm under-estimated the dose for one patient treated on nose, by as much as 28.5% for the patient plan and 12.9% for the QA plan. For this patient, a calculation based on an output look-up table was within 2% of the measurement.

Conclusion: The measurements indicate that the eMC algorithm predicts dose reasonably well for QA plans using the same homogeneous phantom. However, for CT-based patient plans, the eMC algorithm generates a larger number of monitor units versus the QA plan. It may reflect the impact of patient surface curvature and tissue inhomogeneity to the eMC algorithm and further investigation is needed. Caution should be taken for eMC patient plan with small field size and extended SSD. A lookup table of output factors based on measurement can be generated as a quick reference for electron dose calculation.